The goal of this ongoing study is to determine how the activities of intracellular compartments are coordinated and how this coordination is maintained during changes in cellular function. Mammary gland was chosen as the model tissue because it undergoes well known hormonally controlled stages, it responds to hormones under precisely controlled in vitro conditions, and its metabolic pathways are more easily evaluated than those of other tissues. During the current grant year we have shown that the transformation in cellular function during the transition from the pregnant to lactating state involves alterations in anion transport across the mitochondrial membrane, in enzymatic activity, and in the flux along metabolic pathways. This functional change also affects the intracellular concentration of metabolic intermediates, but the response to these changes is not altered. Our immediate aim is to determine how changes in such diverse activities are initiated and modulated in such a manner that a cell assumes a new, hormonally induced, role.